1. Field of the Invention
The present invention generally relates to a self-refresh control circuit, and more specifically, to a self-refresh control circuit simplified with a division value generated in an Extended Mode Register Set (hereinafter, referred to as “EMRS”) without a limiter used in a Temperature Compensated Self Refresh (hereinafter, referred to as “TCSR”).
2. Description of the Prior Art
Generally, a self-refresh operation refers to a self-operation performed with a predetermined cycle, that is, a basic cycle, in a volatile memory device such as DRAM in order to maintain data stored in a memory cell at a standby state.
It is important to reduce self-refresh current at the standby state in the DRAM used for a portable device.
Methods for reducing the self-refresh current includes a Partial Array Self Refresh (hereinafter, referred to as “PASR”), Temperature Compensated Self Refresh (hereinafter, referred to as “TCSR”) and a Deep Power Down Mode (hereinafter, referred to as “DPD”). Here, the PASR and the TCSR are programmed by a user.
In the conventional programmed TCSR, a self-refresh cycle is changed depending on a temperature set by a user. Since data retention time of the device at low temperature more increases than at high temperature, the self-refresh cycle is set to be longer when the DRAM is used at low temperature, thereby reducing power consumption.
However, programmed EMRS-TCSR is a limited method because the operation of the DRAM cannot be secured when the usage temperature of actual product is over the programmed temperature range.
In order to improve the above-described problem, an auto-TCSR is used. The auto-TCSR is to automatically regulate a refresh cycle depending on temperature not by setting temperature by a user but by monitoring temperature in a chip. Specifically, the auto-TCSR having a temperature sensor in the memory chip is an on-die TCSR.
In the on-die TCSR, current flowing in a diode is changed depending on temperature. Here, the refresh cycle is set to be long as current flowing in a diode becomes small at low temperature. Since the change quantity of the refresh cycle depending on temperature is larger in the conventional circuit than in the EMRS, the refresh cycle becomes longer than the refresh limit of the actual device at low temperature, which results in operating errors. Therefore, a limiter is required to prevent the refresh cycle from being longer than a predetermined cycle at low temperature.
In case of the conventional on-die TCSR, a basic cycle which decreases with increase of temperature is generated, a final self-refresh cycle is set after division is selected in consideration of distribution of the chip.
However, since the distribution of the basic cycle is dispersive, a selected division value is differentiated.
Furthermore, the circuit is complicated because the limiter is used to prevent generation of bit fail due to long basic cycle at low temperature so that the basic cycle may not be longer than a predetermined cycle.